Power Semiconductor Devices, Methods, and Structures with Embedded Dielectric Layers Containing Permanent Charges

ABSTRACT

Power devices using refilled trenches with permanent charge at or near their sidewalls. These trenches extend vertically into a drift region.

CROSS-REFERENCE

Priority is claimed from U.S. application 61/168,794 filed 13 Apr. 2009,which is hereby incorporated by reference.

BACKGROUND

The present application relates to power devices, and more particularlyto power devices with vertical or lateral current flow through a driftregion.

Note that the points discussed below may reflect the hindsight gainedfrom the disclosed inventions, and are not necessarily admitted to beprior art.

High voltage devices such as Diodes, JFETs, IGBTs and MOSFETs are widelyused as switching devices in many electronic applications. In order tominimize the conduction power loss in power MOSFETs, it is desirable tohave a low specific on-resistance (R_(s)p or R*A), which is the productof the on-resistance of the MOSFET multiplied by the active die area. Ingeneral, the on-resistance of a power MOSFET is dominated by the channeland drift region resistances. In recent years, the so-calledsuperjunction structure has been developed to reduce the drift regionresistance. A superjunction device is constructed by paralleling highlydoped alternating p-type and n-type layers or pillars. For a givenbreakdown voltage, the doping concentration of the n-type pillars (then-type drift region), can be significantly higher than that of aconventional drift region, provided that the total charge of the n-typepillar is balanced with the charge in the p-type pillar. In order tofully realize the benefit of the superjunction technology, it isdesirable to pack many pillars in a given area to achieve a lower Rsp.However, the minimum widths of the n-type and p-type pillars which canbe reached in device manufacturing set a limit on the minimum cell pitchas well as on scaling of the device.

Recently, some of the present inventors have addressed this issue byincorporating permanent positive charge (Q_(F)) in trenches filled withdielectric material such as silicon oxide. FIGS. 1 and 2 generallyillustrate these concepts, and more detail can be found in published USApplications 20080191307, 20080164516, and 20080164518, all of which arehereby incorporated by reference.

FIG. 1 shows an example of a diode structure, in which frontsidemetallization 103 and backside metallization 102 provide low-resistanceconnections for cathode and anode, respectively. The semiconductor mass,in this example, is silicon, and has a p-type epitaxial layer 112 overan n+ substrate 110. A P+ doped region 114 provides good ohmic contactto the frontside metallization 103. Trenches 120 are filled (in thisexample) with a dielectric material 122, typically silicon dioxide.Permanent charge 124 is present near the sidewalls of the trenches 120.For example, such charge can be introduced by angle-implanting cesiumions into a thin grown oxide on the sidewalls of the trenches 120 beforethey are filled.

FIG. 2 shows one example of a trench transistor as described in thepublished applications referenced above. Here insulated polysilicon gateelectrodes 230 are present in the upper parts of trenches 120, and thesemiconductor structure near the front surface includes a p-type bodyregion 240 which is contacted by a p+ body contact diffusion 242, aswell as an n+ source region 244. Positive permanent charge 124 ispresent near the trench sidewalls, and provides improved chargebalancing when the epitaxial layer 112 is depleted under reverse bias.

The permanent charge also forms an induced drift region by forming aninversion layer along the interface between the oxide and the P-typelayer. Using this new concept, the scaling limitation due tointer-diffusion of p-type pillar and n-type pillar can be eliminated.Consequently, a small cell pitch and high packing density of pillars(and of channel area) can be realized, to thereby reduce the devicetotal on resistance and Rsp.

As the trench depth increases and cell pitch reduces, the trench depthto width aspect ratio increases. High aspect ratio trenches are moredifficult to fill with dielectric material, and manufacturing the devicebecomes a problem. Furthermore, as the cell pitch is reduced and thecell density increases, the associated intrinsic capacitances of thedevice also increase undesirably. These include the gate-to-source andgate-to-drain capacitances C_(gs) and C_(gd). The correspondinggate-source and gate-drain charges Q_(gs) and Q_(gd) also increase.

SUMMARY

The present application discloses a number of new approaches toovercoming the problems of prior art devices.

One class of new devices includes refilled trenches. In variousembodiments the trench fill material can be epitaxially grown upwardfrom the underlying semiconductor substrate, or can be a depositedsemiconducting or semi-insulating material.

The present application also discloses a variety of new devicestructures, which generally, in various ways, include buried or embeddedlayers of dielectric material which contain permanent charges.Preferably these charges are positioned and oriented in proximity tocarrier trajectories in the drift region.

Another group of inventions provides a new edge termination structurefor a power device array.

The disclosed innovations, in various embodiments, provide one or moreof at least the following advantages. However, not all of theseadvantages result from every one of the innovations disclosed, and thislist of advantages does not limit the various claimed inventions.

-   Reduced specific on-resistance (R_(SP));-   Reduced heat dissipation;-   Improved energy efficiency;-   Reduced parasitic capacitances;-   Reduced charge stored in parasitic capacitances;-   Improved switching efficiency;-   Improved switching speed;-   Reduced cost for given performance parameters.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

FIG. 1 and FIG. 2 schematically show diode and MOSFET structurespreviously proposed by various ones of the present inventors.

FIG. 3A shows a diode trench structure, and FIG. 3B shows another diodestructure.

FIGS. 4A, 4B, 4C, 4D, 5A, 5B, 5C, 5D, 6A, 6B, 7A, and 7B show manydifferent trench transistor structures, according to various disclosedinnovative embodiments.

FIGS. 8A, 8B, 9A, 9B, and 9C show many different planar andquasi-vertical transistor structures.

FIGS. 10A, 10B, 10C, 10D, 10E, 11A, 11B, and 11C show various examplesof trench gate transistors, according to various different innovativeembodiments, which include buried dielectric layers with permanentelectrostatic charges.

FIGS. 12A, 12B, and 12C show various embodiments of planar DMOStransistors, according to various different innovative embodiments,which include buried dielectric layers with positive permanentelectrostatic charges.

FIGS. 13A and 13B show various other embodiments of planar CMOScompatible transistors, according to various different innovativeembodiments, which include buried dielectric layers with permanentelectrostatic charges, and also include shield layers.

FIG. 14 shows an example embodiment of a quasi-vertical device,according to various different innovative embodiments, which includeburied dielectric layers with positive permanent electrostatic charges.

FIGS. 15A-15L show one possible process flow to fabricate trench gatetransistors that contain trenches with SIPOS TFL that contain permanentcharge.

FIGS. 16A-16E show a different process flow, in which trenches arefilled with an epitaxial silicon trench filling layer.

FIGS. 17A-17D show one possible flow of fabricating buried dielectriclayers that contain permanent charge.

FIGS. 18 and 19 show various termination structures which combinesynergistically with the above device embodiments.

FIGS. 20 and 21A-21B show various layouts for the edge of the array,using graduated trench diameters.

FIGS. 22A, 22B, and 22C are plan views which show various alternativesfor the layout of the refilled trenches and of the device channels.

DETAILED DESCRIPTION OF SAMPLE EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to presently preferred embodiments(by way of example, and not of limitation). The present applicationdescribes several inventions, and none of the statements below should betaken as limiting the claims generally.

The present application describes several new device and fabricationconcepts, and many different ways to implement them. A number of theseconcepts and embodiments will be described in detail, but it must beremembered that the new concepts described here include some verybroadly applicable points.

One class of embodiments describes devices which include trenches withwalls covered by a dielectric material such as silicon dioxide thatcontains permanent charges and a Trench Fill Layer (TFL). A TFL isformed, for example, by using techniques such as Selective EpitaxialGrowth (SEG). Alternatively, a TFL can be formed of a semi-insulatingmaterial such Semi-Insulating-Poly-Silicon (SIPOS), or of otherprocess-compatible semiconducting or semi-insulating materials. Ofcourse, the conduction properties of the trench fill layer will beaffected by the choice of the TFL (and of its deposition conditions).Semiconductor material formed by Selective Epitaxial Growth (“SEG”) willoften have a higher level of crystal defects.

The disclosed device structures where the TFL is a silicon epitaxiallayer (e.g. by using SEG) have the advantage that additional currentpaths are provided along the interfaces of the dielectric layers(containing permanent charge) and the epitaxial TFL layers. Therefore,current conduction occurs along all interfaces of a dielectric layer,rather than along one interface only (as in the case of a device using adielectric or a semi-insulating material TFL). The additional currentpaths result in lower R_(SP). Furthermore, the width of the dielectriclayer (such as silicon dioxide) can be made very thin (e.g. on the orderof 10 nm) and therefore results in a lower R_(SP).

The present application discloses a number of new device structures,which generally, in various ways, include buried or embedded layers ofdielectric material which contain permanent charges.

Buried dielectric layers formed for example by silicon dioxide can becompletely surrounded by silicon material. In the on-state, inversionlayers are formed along the dielectric layers' edges due to theexistence of permanent charges. These inversion layers provide currentconduction paths and result in lower Rsp. Such structures can befabricated for example using techniques such as Selective EpitaxialGrowth (SEG) and permanent charges introduced using implanted Cesiumions.

FIG. 3A depicts a diode or voltage blocking trench structure built on ap-type epitaxial layer (P0) formed on top of an N+ substrate 110. Thetrench walls are covered by dielectric material 322, such as silicondioxide, which contains permanent positive charge 124. The TFL 323 canbe formed, for example, by Selective Epitaxial Growth (SEG). As shown inFIG. 3A, the TFL includes three different doped regions. At the bottomthere is an n+ region 323A connected to the N+ substrate 110, in themiddle a p-type region 323B, and on the top a p+ region 323C. The TFLlayer 323 preferably completely fills the trench 120. The doping in themiddle p region 323B of the silicon TFL 323 can be the same as ordifferent from that of the starting material epitaxial layer P0. Thedoping of TFL middle p-region 323B is adjusted such that the diodestructure can block a high voltage at reverse bias.

FIG. 3B depicts a diode or voltage blocking structure that is generallysomewhat similar to that of FIG. 3A, except that the TFL layer 323 isformed by a high resistivity material 325, such as undoped polysilicon,or of a semi-insulating material such as Semi-Insulating-Poly-Si(SIPOS). The high resistivity polysilicon can, for example, be formed ofp-n neutralized polysilicon to reduce the temperature dependence ofmaterial characteristics.

FIG. 4A depicts a sample embodiment of a trench MOSFET structure builton a p-type epitaxial layer (P0) formed on top of an N+ substrate 110.The trench walls are covered by a dielectric material 322, such assilicon dioxide, which contains permanent positive charge 124. Thetrenches in FIG. 4A are filled with a Trench Fill Layer (TFL) 323, whichcan be formed, for example, by Selective Epitaxial Growth (SEG) ofsilicon. As shown in FIG. 4A, a layer 410 of dielectric material ispresent between the p-type region P1 of the TFL layer 323 and theconducting gate material 230.

The Selective Epi layer P (for example in FIG. 4A) can be connected tothe P+ (Source contact) in the 3rd dimension (perpendicular to the planeof the cross section of FIG. 4A), e.g. in accordance with the geometriesshown in FIG. 22A.

The top end of the TFL layer 323 is preferably electrically connected tothe source as shown in one embodiment in FIG. 4B.

FIG. 4C depicts another embodiment of trench MOSFET structure built onp-type epitaxial layer (P0) formed on top of a N+ substrate. The trenchwalls are covered by a dielectric material, such as silicon dioxide,which contains permanent positive charge. The trenches in FIG. 4C arefilled with a semi-insulating Trench Fill Layer (TFL). The TFL can beformed, for example, by using SIPOS.

The TFL layer (for example in FIG. 4C) can be connected to the Sourcemetal in the 3rd dimension (perpendicular to the plane of FIG. 4C crosssection).

In another class of embodiments, the top end of the TFL layer 323 can beelectrically connected to the source as shown in one embodiment in FIG.4D. Note that two kinds of trench are used in this embodiment. Note thathigh-resistivity material, such as SIPOS, is used in this embodiment,since otherwise the material in the trench 120′ would provide a directshort from source to drain.

FIG. 5A shows another embodiment of a trench MOS transistor which isgenerally somewhat similar to that shown in FIG. 4A, except that thetrench dielectric layer extends above the silicon surface to form a“nailhead” shape.

FIG. 5B shows another embodiment of a trench MOS transistor which isgenerally somewhat similar to that shown in FIG. 4C, but with a trenchdielectric layer above the silicon surface.

FIG. 5C shows another embodiment of a trench MOS transistor which isgenerally somewhat similar to that shown in FIG. 5A, except with atrench source contact.

FIG. 5D shows another embodiment of a trench MOS transistor, which isgenerally somewhat similar to that shown in FIG. 5B, except that atrench source contact is used.

FIG. 6A shows another embodiment of a trench MOS transistor, which isgenerally somewhat similar to that shown in FIG. 4A, except that aN-drift region is provided on top of the N+ substrate.

FIG. 6B shows another embodiment of a trench MOS transistor, which isgenerally somewhat similar to that shown in FIG. 4C except that anN-drift region is provided on top of the N+ substrate.

FIG. 7A shows another embodiment of a trench MOS transistor, which isgenerally somewhat similar to that shown in FIG. 6A, except that thetrench depth extends only to the N-drift region.

FIG. 7B shows another embodiment of a trench MOS transistor, which isgenerally somewhat similar to that shown in FIG. 6B, except that thetrench depth extends only to the N-drift region.

In the devices shown in FIGS. 5A-5B, 6A-6B, and 7A-7B the bottom end ofa silicon (or semi-insulating) TFL is connected to the drain while thetop layer is preferably electrically connected to the source at certainareas of the device (not shown).

FIG. 8A shows a sample embodiment of a planar DMOS transistor withsemi-insulating TFL layer (such as SIPOS).

FIG. 8B shows a planar DMOS transistor similar to that shown in FIG. 8Abut with an additional surface n-layer and N-drift layer.

FIGS. 9A, 9B, and 9C show other embodiments of planar DMOS transistorswith semi-insulating TFL and conductive gate shield layer formed forexample of heavily doped polysilicon layer and is preferablyelectrically connected to the source potential. The gate shield layerresults in reducing gate-drain capacitance Cgd.

It is also worth noting that in the devices shown in FIGS. 8A-8B and9A-9C the bottom end of a semi-insulating TFL is connected to the drainwhile the top layer is preferably electrically connected to the sourceat certain areas of the device (not shown).

In the following embodiments another class of new device structures aredisclosed that have buried or embedded dielectric layers that containpermanent charges. Layers of dielectric material such as silicon dioxidecontaining permanent charges are buried inside silicon regions andcurrent conduction occurs along all interfaces of a dielectric layer. Inthese structures the width of the dielectric layer containing permanentcharge can be made very thin (for example 10-100 nm) and therefore morecurrent conduction paths can be formed for the same active area of thedevice. The current conduction paths are formed by inversion layerscreated along the interfaces of the buried dielectric layers with p-typesilicon region due to the existence of positive permanent charges. Suchstructures therefore offer the significant advantage of reduced R_(SP).Such structures can be fabricated for example using techniques such asSelective Epitaxial Growth (SEG) and permanent charges formed, forexample, by using implanted Cesium ions.

FIG. 10A depicts one such embodiment of a trench gate MOSFET with burieddielectric layers that contain positive permanent charges. In theon-state electron current flows from the n+ source through the MOSchannel to the n-region underneath the trench gate oxide and thenthrough the inversion layers along the dielectric layer interface withsilicon created due to the permanent charges. In FIG. 10A the doping ofregions PO and P1 can be the same or different. An example of acorresponding layout, which can be used with this device structure, isshown in FIG. 22B.

FIG. 10B shows another embodiment of a trench gate MOSFET which isgenerally similar to that shown in FIG. 10A, except that there is anadditional n layer located on top of the N+ substrate.

FIG. 10C shows another embodiment of a trench gate MOSFET which isgenerally similar to that shown in FIG. 10A except that there is anadditional shield layer of conductive material such as doped polysiliconlocated below the gate electrode to reduce Cgd. The shield layer ispreferably electrically connected to the Source potential.

FIG. 10D shows another embodiment of a trench gate MOSFET which isgenerally similar to that shown in FIG. 10B except that the n regionsbelow the trench gated overlap .

FIG. 10E shows another embodiment of a trench gate MOSFET which isgenerally similar to that shown in FIG. 10C except that the n regionsbelow the trench gated overlap.

FIG. 11A depicts another embodiment of a trench gate MOSFET with burieddielectric layers that contain positive permanent charges with acontinuous n-layer below the trench gate and using a trench sourcecontact.

Note that the original epitaxial doping and mesa width can be differentfrom those of the Selective Epi layer. A particularly advantageous caseis when the product of doping times width is equal for the two.

Note that, in FIG. 11A, the buried dielectric layers, which containcharge, can be misaligned to the surface geometry.

An example of a corresponding layout, which can be used with this devicestructure, is shown in FIG. 22C.

FIG. 11B shows another embodiment of a trench gate MOSFET which isgenerally similar to that shown in FIG. 11A except that there is anadditional shield layer of conductive material such as heavily dopedpolysilicon located under the gate electrode that reduces Cgd. Theshield layer is preferably electrically connected to the Sourcepotential.

FIG. 11C shows another embodiment of a trench gate MOSFET which isgenerally similar to that shown in FIG. 11B except that some of theburied dielectric layers contact the trench gate dielectric.

FIGS. 12A, 12B, and 12C show various embodiments of planar DMOStransistors with buried dielectric layers that contain positivepermanent charges. Similarly the buried dielectric layers provide lowerRSP since more current conduction paths are formed by the inversionlayers along the edges of the buried layers due to the existence ofpermanent charges.

FIGS. 13A and 13B show various other embodiments of planar DMOStransistors with buried dielectric layers that contain positivepermanent charges and shield layers.

It should be noted that in FIGS. 11A, 11B, 11C, 12A, 12B, 12C, 13A, and13B that the P epitaxial layer can be connected to the P+ (Sourcecontact) in the 3rd dimension.

FIG. 14 shows another embodiment of a quasi-vertical device where the N+substrate is replaced by an N+ buried layer formed on top of aP-substrate. The N+ buried layer forms the drain of the device and canbe connected to a surface drain contact through a N+ deep sinker or atrench filled with conductive material such as tungsten (not shown). Theadvantage of this embodiment is that the device can be integratedmonolithically with other components such as CMOS, Bipolar transistors,resistors, capacitors, etc.

FIGS. 10-14 show that the buried dielectric layers at various locationswith respect to the source and gate. The buried dielectric layers can beplaced such that the device performance is optimized.

FIGS. 15A-15L show one possible process flow to fabricate trench gateMOSFET that contain trenches with SIPOS TFL that contain permanentcharge.

FIGS. 16A-16E show a variation to the previous flow that includestrenches filled with an epitaxial silicon TFL.

FIGS. 17A-17D show one possible flow of fabricating buried dielectriclayers that contain permanent charges. The remaining steps to fabricatea MOSFET transistors such as shown in FIG. 11A-11C or 12A-12C areconventional and known to those familiar with the art.

FIG. 18 shows an embodiment where a wider trench is filled withsemi-insulating material such as SIPOS. The trench edge can beoptionally contacted by a conducting material such metal connected tothe drain potential.

FIG. 19 shows another termination embodiment wherein the wide trenchcontains SIPOS and is in contact with the N+ substrate, and thetermination trench is not completely filled. The SIPOS layer can becovered by an insulating material such as oxide (not shown).

A high voltage edge termination for a device may also be simultaneouslyfabricated. A gradually decreasing charge depth is obtained by formingopenings having decreasing width as the distance from the active areaincreases. Since the atoms that provide the charge in the oxide sidewallmay be implanted, choice of the implant angle to just reach the bottomof the widest trench also results in implanting the atoms to a depththat is less than that of the trench for any trench having a smallerwidth, as shown in FIG. 20. The spacing between trenches may also beadjusted to provide the charge balance required the breakdown voltage,as also shown in FIG. 20.

The layout of the trenches used for the termination region may varygreatly while achieving the same result. Two such layouts are shown inFIGS. 21A-21B. FIG. 21A shows the use of long, rectangular trenchesalong the immediate edges of the active region, while circular trencheshaving different sizes are used in the corner.

FIG. 21B shows the use of circular trenches having different sizes alongthe entire perimeter of the device. Other trench shapes may also beused, such as squares, hexagons, octagons, ovals, and irregular shapes,as long as the charge balance requirements are met.

The doping levels needed to achieve high breakdown and low-resistanceare governed by the well known charge balance condition. The specificelectrical characteristics of devices fabricated using the methodsdescribed in this disclosure depend on a number of factors including thethickness of the layers, their doping levels, the materials being used,the geometry of the layout, etc. One of ordinary skill in the art willrealize that simulation, experimentation, or a combination thereof canbe used to determine the design parameters needed to operate asintended.

According to some (but not all) disclosed embodiments, there isprovided: A semiconductor device, comprising: a current-controllingstructure at a first surface of a semiconductor mass; a semiconductivedrift region extending down into said semiconductor mass; saidcurrent-controlling structure and said drift region being jointlyconnected in series between a first source/drain region at said firstsurface, and a second source/drain region which is not connected to saidfirst source/drain region; and trenches extending down into saidsemiconductor mass, at least some ones of said trenches being at leastpartially filled with a trench-filling material which is not aninsulator, and which is electrically connected to said secondsource/drain region.

According to some (but not all) disclosed embodiments, there isprovided: A semiconductor power device, comprising: a layeredsemiconductor structure which includes at least an upper semiconductorportion and a lower semiconductor portion; said upper portion includinga current-controlling structure, and said lower portion including adrift region; said current-controlling structure and said drift regionbeing jointly connected in series between a first source/drain region atsaid first surface, and a second source/drain region; and trenchesextending down into said semiconductor mass, at least some ones of saidtrenches being at least partially filled with a trench-filling materialwhich is not an insulator, and which is electrically connected to saidsecond source/drain region.

According to some (but not all) disclosed embodiments, there isprovided: A semiconductor power device, comprising: acurrent-controlling structure at a first surface of a semiconductormass; a semiconductive drift region extending down into saidsemiconductor mass; said current-controlling structure and said driftregion being jointly connected in series between a first source/drainregion at said first surface, and a second source/drain region at asecond surface of said semiconductor mass; and trenches extending downthrough said drift region into said semiconductor mass, at least someones of said trenches being at least partially filled with atrench-filling material which is not an insulator, and which iselectrically connected to said second source/drain region; said trenchesand said current-controlling structure having completely independentlateral alignment.

According to some (but not all) disclosed embodiments, there isprovided: A semiconductor power device, comprising: an array of activedevice cells, including both an upper array of current-controllingstructures, and a lower array of permanently charged trenches within asemiconducting drift region; said trenches individually containingpermanent electrostatic charge at or near sidewalls thereof; and atransitional structure surrounding said array, said transitionalstructure including at least some trenches which are fabricated in thesame steps as said trenches in said array, but which have smallermaximum width, and a different ratio of charge density between upper andlower portions of the walls of said trenches.

According to some (but not all) disclosed embodiments, there isprovided: A semiconductor voltage blocking structure, comprising: alayered semiconductor structure which includes at least an uppersemiconductor portion and a lower semiconductor portion; said upperportion including a first anode/cathode diffusion, and said lowerportion including a drift region; said drift region being electricallyinterposed in series between said first anode/cathode region, and asecond anode/cathode region at a second surface of said semiconductormass; and trenches extending down into said semiconductor mass, at leastsome ones of said trenches being at least partially filled with asemiconductive trench-filling material which conducts current inparallel with portions of said lower portion outside said trench.

According to some (but not all) disclosed embodiments, there isprovided: A method for operating a power device which switches betweenan ON state and an OFF state, comprising: controlling ON-state currentby use of a current-controlling structure at a first surface of asemiconductor mass; said ON-state current flowing through a driftregion, to provide a controllable electrical connection between a firstsource/drain region, at said first surface, and a second source/drainregion which is not otherwise connected to said first source/drainregion; and blocking current, in the OFF state, using said drift regionin conjunction said current-controlling structure; wherein bottomlesstrenches extend into said drift region, and at least some ones of saidtrenches are at least partially filled with a trench-filling materialwhich is not an insulator, and which is electrically connected to saidsecond source/drain region.

According to some (but not all) disclosed embodiments, there isprovided: A method for operating a power device, comprising: controllingturn-on and turn-off using an array of semiconductor devices; whereinsaid devices include trenches which have a first minimum width, andwhich have been angle-implanted with ions which provide immobileelectrostatic charge; and providing a graded transition of voltageoutside said array, using peripheral trenches which have graduatedminimum widths, including widths which are less than said first minimumwidth; wherein narrower ones of said peripheral trenches have a lowerdensity of said ions in bottom portions thereof, as compared with upperportions thereof, than do wider ones of said trenches.

According to some (but not all) disclosed embodiments, there isprovided: A method for fabricating a power device, comprising the stepsof: etching one or more trenches vertically into a semiconductor mass;forming a thin dielectric layer on sidewalls of said trenches;introducing immobile electrostatic charge into said thin dielectric;anisotropically etching through the bottoms of said trenches into saidsemiconductor mass; and at least partially filling said trenches with asemiconductor material which is electrically connected to saidsemiconductor

According to some (but not all) disclosed embodiments, there isprovided: A method for fabricating a power device, comprising the stepsof: etching one or more trenches vertically into a semiconductor mass;forming a thin dielectric layer on sidewalls of said trenches;introducing immobile electrostatic charge into said thin dielectric;anisotropically etching through the bottoms of said trenches into saidsemiconductor mass; and at least partially filling said trenches with asemiconductor material which is electrically connected to saidsemiconductor mass at the bottoms of said trenches.

According to some (but not all) disclosed embodiments, there isprovided: A method for fabricating a power device, comprising the stepsof: etching a plurality of trenches vertically into a semiconductormass, both in the location of an array of active devices and also in aperiphery around said array of active devices; said trenches in saidperiphery having graded widths which become progressively smalleroutside said array; and angle-implanting immobile ions into saidtrenches to thereby create confer immobile electrostatic charge attrench sidewalls which are thereby implanted; said angle-implanting stepbeing performed at an angle where bottoms of the narrowest trenches arenot exposed; wherein said trenches in said periphery have graduatedelectrical properties outside said array.

Modifications and Variations

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given. It is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

The doping levels needed to achieve high breakdown and low-resistanceare governed by the well known charge balance condition. The specificelectrical characteristics of devices fabricated using the methodsdescribed in this disclosure depend on a number of factors including thethickness of the layers, their doping levels, the materials being used,the geometry of the layout, etc. One of ordinary skill in the art willrealize that simulation, experimentation, or a combination thereof canbe used to determine the design parameters needed to operate asintended.

While the figures shown in this disclosure are qualitatively correct,the geometries used in practice may differ and should not be considereda limitation in anyway. It is understood by those of ordinary skill inthe art that the actual cell layout will vary depending on the specificsof the implementation and any depictions illustrated herein should notbe considered a limitation in any way. Both cellular and interdigitatedsurface geometries are well known to one of ordinary skill in the art.

While only diodes and MOSFET transistors are shown here, other devicessuch as IGBTs or JFETs can also be realized using techniques which areknown by those of ordinary skill in the art.

While only n-channel MOSFETs are shown here, p-channel MOSFETs arerealizable with this invention simply by changing the polarity of thepermanent charge and swapping n-type and p-type regions in any of thefigures. This is well known by those of ordinary skill in the art. It isalso understood that this invention is also valid if the oppositepolarity of the permanent charge i.e. negative charge and thesemiconductor materials types are used.

It should be noted in the above drawings the positive permanent chargewas drawn for illustration purpose only. It is understood that thischarge can be in the dielectric (oxide) layer, at the interface betweenthe silicon and oxide, inside the silicon layer, or can be implementedwith a combination of these cases.

The examples of Trench Fill Layer most discussed above are SelectiveEpitaxial Growth (SEG) or Semi-Insulating-Poly-Silicon (SIPOS). Ofcourse, the conduction properties of the trench fill layer will beaffected by the choice of the TFL (and of its deposition conditions).For example, semiconductor material formed by Selective Epitaxial Growthwill often have a higher level of crystal defects than a normalepitaxial starting structure. As the SIPOS example shows, the materialinside the trenches does not have to be the same as the originalsemiconductor material.

It should also be understood that numerous combinations of the aboveembodiments can be realized. \While only diodes and MOSFET transistorsare shown here, other devices such as IGBTs, JFETs can also be realizedusing techniques which are known by those of ordinary skill in the art.

For another example, integrated structures can be built using a deeptrench contact or a sinker diffusion, as mentioned above. In such casesa frontside drain contact can be made to the sinker diffusion.

While only n-channel MOSFETs are shown here, p-channel MOSFETs arerealizable with this invention simply by changing the polarity of thepermanent charge and swapping n-type and p-type regions in any of thefigures. This is well known by those of ordinary skill in the art. It isalso understood that this invention is also valid if the oppositepolarity of the permanent charge i.e. negative charge and thesemiconductor materials types are used.

Additional general background, which helps to show variations andimplementations, may be found in the following U.S. and PCTapplications, all of which, and all their priority applications, arehereby incorporated by reference: WO/2008/039459, US20080073707,US20080191307 US20080164516 US20080164518 US20080164520 US20080166845W02009/102684; US20090206924; WO/2009/102651, US20090206913;US20090294892; U.S. Ser. Nos. 12/431,852; 12/394,107; 12/432,917;12/431,005; 12/369,385; 12/545,808; 12/626,523; 12/626,589. Applicantreserves the right to claim priority from these applications, directlyor indirectly to even earlier applications, in all countries where suchpriority can be claimed.

None of the description in the present application should be read asimplying that any particular element, step, or function is an essentialelement which must be included in the claim scope: THE SCOPE OF PATENTEDSUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none ofthese claims are intended to invoke paragraph six of 35 USC section 112unless the exact words “means for” are followed by a participle.

The claims as filed are intended to be as comprehensive as possible, andNO subject matter is intentionally relinquished, dedicated, orabandoned.

1-59. (canceled)
 60. A method for operating a power device whichswitches between an ON state and an OFF state, comprising: controllingON-state current by use of a current-controlling structure at a firstsurface of a semiconductor mass; said ON-state current flowing through adrift region, to provide a controllable electrical connection between afirst source/drain region, at said first surface, and a secondsource/drain region which is not otherwise connected to said firstsource/drain region; and blocking current, in the OFF state, using saiddrift region in conjunction said current-controlling structure; whereinbottomless trenches extend into said drift region, and at least someones of said trenches are at least partially filled with a trenchfilling material which is not an insulator, and which is electricallyconnected to said second source/drain region; wherein saidcurrent-controlling structure comprises a source, a body, and a gatewhich is capacitively coupled to control inversion of a nearby portionof said body.
 61. (canceled)
 62. The method of claim 60, wherein saidbottomless trenches have immobile electrostatic charge in proximity tosidewalls thereof.
 63. The method of claim 60, wherein said trenchfilling material is silicon.
 64. The method of claim 60, wherein saidtrench filling material is a substantially crystalline semiconductormaterial.
 65. The method of claim 60, wherein said trench fillingmaterial is substantially the same semiconductor material as parts ofsaid drift region outside said trenches.
 66. The method of claim 60,wherein said semiconductor mass is monocrystalline silicon.
 67. A methodfor operating a power device, comprising: controlling turn-on andturn-off using an array of semiconductor devices; wherein said devicesinclude trenches which have a first minimum width, and which have beenangle-implanted with ions which provide immobile electrostatic charge;and providing a graded transition of voltage outside said array, usingperipheral trenches which have graduated minimum widths, includingwidths which are less than said first minimum width; wherein narrowerones of said peripheral trenches have a lower density of said ions inbottom portions thereof, as compared with upper portions thereof, thando wider ones of said trenches.
 68. The method of claim 67, wherein saiddevices are trench transistors.
 69. The method of claim 67, wherein saidtrenches are bottomless. 70-97. (canceled)